1. Field of the Invention
The present invention relates to a developer for developing an electrostatic image, that is used for converting an electrostatic latent image to a visible image in image forming processes such as electrophotography, electrostatic recording and electrostatic printing. It also relates to an image forming method, and electrophotographic apparatus, an apparatus unit and a facsimile apparatus that make use of the developer.
More particularly, the present invention relates to a developer for developing an electrostatic image, that is used in electrophotographic processes comprising a charging step of bringing a charging member to which a voltage has been externally applied, into contact with an electrostatic image bearing member to effect electrostatic charging, and a developing step of developing an electrostatic image by using a developer; and an image forming method, an electrophotographic apparatus, an apparatus unit and a facsimile apparatus that make use of the developer.
2. Related Background Art
Corona dischargers are hitherto known as charging means in electrophotographic apparatus and so forth. The corona dischargers, however, have the problems that a high voltage must be applied thereto and ozone is produced in a large quantity.
It is recently studied to omit a corona discharger and to use a contact charging means. Stated specifically, this is a means in which a voltage is applied to a conductive roller serving as a charging member, and then the roller is brought into contact with a photosensitive member which is a member being charged, so that the surface of the photosensitive member is electrostatically charged to a given potential. Use of such a contact charging means makes it possible to apply a lower voltage than the use of the corona dischargers and to decrease the generation of ozone.
For example Japanese Patent PubIication No. 50-13661 proposes to use a roller comprising a mandrel covered with a dielectric material such as nylon or polyurethane rubber so that a photosensitive sheet can be electrostatically charged at a low voltage.
The roller comprising a mandrel covered with nylon, however, has no elasticity such as rubber, and hence the roller can not be kept in sufficient contact with the member being charged, so that faulty charging tends to occur. On the other hand, covering the mandrel with polyurethane rubber may cause exudation of a softening agent with which the polyurethane rubber is impregnated, and has involved the problem that, when a photosensitive member is used as the member being charged, the roller tends to stick to the photosensitive member at the former's portion coming into contact with the latter when the photosensitive member is stopped, or that the region corresponding to the part where both had been stuck in contact causes unfocused images. Once the softening agent in the rubber material of the roller has exuded and stuck to the surface of the photosensitive member, the photosensitive member exhibits a low resistivity which causes smeared images (i.e., a leak of charges of an electrostatic image, on the surface of the photosensitive member). In extreme instances either becomes impossible to use the apparatus or the toner remaining on the surface of the photosensitive member then sticks to the roller surface to cause a filming phenomenon. In the event that the toner has stuck to the roller surface in a large quantity, the roller surface turns insulative, resulting in a loss of the charging ability of the roller and a non-uniform charge on the surface of the photosensitive member, which adversely affects images.
This is due to the fact that the charging member (the roller) strongly presses the developer against the surface of the photosensitive member and hence the remaining developer sticks to the Charging member or the surface of the member being charged and also the surface of the charging member and the surface of the member being charged tend to be damaged or scraped.
In the contact charging means, a direct current or a direct current overlaid with an alternating current is applied to the charging member. In such an instance, abnormal charge or flying movement of the remaining developer particularly having a small particle diameter and a light-weight is repeated in the surrounding area of the part at which the charging member and a photosensitive drum come into contact each other. Hence, this area is in such a state that the remaining developer is electrostatically attracted to, or embedded in, the surface of the charging member or photosensitive drum. This is quite different from the case where a noncontact charging means comprised of the conventional corona discharger is used.
Meanwhile, in recent years, copiers, laser printers, etc. which are small-sized, inexpensive and of personal use have come into use. In these small-sized machines, a cartridge system in which a photosensitive member, a developing device and a cleaning device are assembled into a unit is used from the standpoint of making them free from maintenance, and it is desired to use as the developer a one-component magnetic developer since the structure of a developing assembly can be simplified.
In order to form a visible image with a good image quality in the method making use of such a one-component magnetic developer, the developer must have a high fluidity and a uniform chargeability. For this purpose, a fine inorganic powder has been hitherto added and mixed to a toner powder. It has been proposed to use as the fine inorganic powder a fine silica powder having been subjected to hydrophobic treatment, as disclosed in Japanese Patent Applications Laid-open No. 46-5782, No. 48-47345, No. 48-47346, etc. For example, a treated fine silica powder is used which is obtained by reacting a fine silica powder with an organic silicon compound such as dimethyldichlorosilane to substitute silanol groups on the particle surfaces of the fine silica powder with organic groups so that the powder is made hydrophobic.
The developer having such a fine inorganic powder, however, tends to cause scratches particularly on the contact charging member and photosensitive member and to cause melt adhesion or filming of the toner to the contact charging member and photosensitive member, in an image forming step at Which the developer is pressed against the photosensitive member by contact charging. In an extreme instance, faulty images tend to be formed.
With regard to addition of fine resin particles to a developer, Japanese Patent Application Laid-open No. 60-186854 proposes to add to a developer, spherical or substantially spherical polymer particles smaller than toner particles.
A developer prepared in the same manner as disclosed therein has been examined to reveal that the developer is less effective for preventing the melt adhesion of toner onto the photosensitive member and, in the apparatus making use of contact charging, the contact charging device is contaminated to tend to cause charge non-uniformity.
Japanese Patent Application Laid-open No. 1-121861 proposes a developer prepared by adding fine organic particles to toner particles containing an ionically cross-linked vinyl polymer as a binder resin. It is noted therein that this developer may preferably comprise spherical fine organic particles.
As methods of fixing a toner image, a contact heating method as typified by a heat roller fixing method is commonly used, and there is a demand for a toner capable of being fixed at a low temperature so that power consumption can be decreased. For this reason, it is proposed to incorporate a resin with a low-molecular component and a high-molecular component so that the low-temperature fixing performance and anti-offset properties can be improved.
When, however, a developer containing the binder resin in which the low-molecular component has been increased for the purpose of low-temperature fixing is used in an image forming apparatus having a contact charging device or a contact transfer means, the following problems tend to arise.
Presence of a large quantity of the low-molecular component in a binder resin brings about so excess grindability of toner particles such that the toner particles tend to be broken to give ultrafine particles because of shear produced during preparation. In a developing device the ultrafine particles slip through a cleaning member and adhere to the contact charging member or contact transfer means to tend to cause faulty charging or faulty transfer in an environment of low temperature and low humidity, and cause the melt adhesion of toner to the surface of the photosensitive member in an environment of high temperature and high humidity.
The ultrafine particles produced as a result of break of toner particles have the same chargeability as the toner particles, and hence inhibit the charging of toner particles to cause a lowering of image density.
In the contact heating method, it is required for the toner to be properly softened and fixed at the heating temperature, and it is also required to prevent occurrence of what is called the offset phenomenon, which is a phenomenon in which part of softened toner adhers to a heating member and the adhered toner is transferred to a transfer sheet to contaminate an image. In order to better prevent this offset phenomenon, it is known to incorporate toner particles with a polyolefin such as a low-molecular polyethylene or polypropylene, as disclosed in Japanese Patent Applications Laid-open No. 49-6523 and No. 50-27546.
Japanese Patent Application Laid-open No. 1-11376 proposes a developer comprising wax-containing toner particles mixed with fine resin particles smaller than the toner particles. It is noted therein that this developer may preferably comprise spherical fine organic particles.
When, however, such a developer comprising the toner containing a polyolefin is used in an image forming apparatus having the contact charging device, the following problems tend to arise.
The polyolefin has so poor a compatibility with the binder resin in the toner particles that a polyolefin having a larger disperse diameter tends to come a free polyolefin when toner is pulverized. Thus, the free polyolefin with a higher resistance, having been developed at an image portion or non-image portion, is transferred from an electrostatic image bearing member to a contact charging member to increase surface resistance, tending to cause faulty charging.
The free polyolefin has a high resistance and is negatively chargeable with respect to iron powder, and hence it makes fogging more serious because of the faulty charging in the case of a negatively chargeable developer and causes a poor fluidity in the case of a positively chargeable developer, tending to bring about blank areas in images and a non-uniformity in image density.
In the meantime, in recent years, with the wide spread of image forming apparatus such as electrophotographic copying machines, their uses have expanded in a great variety, and demands for their image quality have become severer. In the copying of images as in conventional documents and books, it is sought to reproduce images in a very fine and faithful state without causing any crushed line images or broken line images even during the copying of fine characters. In particular, in an instance in which a latent image formed on a photosensitive member provided in the image forming apparatus is a line image with a line width of 100 .mu.m or less, the fine-line reproduction is commonly poor and no satisfactory sharpness of the line image has been achieved. Recently, in an image forming apparatus such as an electrophotographic printer making use of digital image signals, a latent image is formed of the assemblage of dots having a given potential, and its solid portion, half-tone portion and light portion are expressed according to changes in dot density. There, however, is a problem when the toner particles are not faithfully applied to the dots and hence the toner particles are not aligned with the dots, that no gradation, of the toner image can be obtained corresponding to the ratio of dot density at a black area to that of a white are a of the digital image. In the case when the resolution is improved by making dot size smaller, in order to improve image quality, it becomes more difficult to achieve the reproduction of a latent image formed of minute dots, tending to give an image having a poor resolution and gradation and also lacking sharpness.
For the purpose of improving image quality, several developers have been hitherto proposed.
Japanese Patent Applications Laid-open No. 1-112253 and No. 2-284158 propose a toner with a small particle diameter having a specific particle size distribution. The smaller particle diameter a toner has, the more uniformly charged the toner particle surfaces must be made. Hence, in order to achieve both a stable charge quantity and a superior fluidity, it is preferred to add as a charge control agent a dye or a derivative of the dye that can give a proper charge quantity when employed in small amounts.
On the other hand, the smaller the particle diameter a toner has, the more it tends to release a free charge control agent during the step of pulverization and the more it tends to cause the inhibition of fluidity or the contamination of members due to a developer. In particular, the charge non-uniformity tends to occur when a member coming into contact with a photosensitive member has been contaminated.
Japanese Patent Application Laid-open No. 1-113762 proposes a developer comprising a mixture to fine acrylic resin particles and toner particles containing a charge control agent. It is noted therein that this developer may preferably comprise spherical fine organic particles.
When, however, such a developer containing a dye or a derivative of the bye as a charge control agent is used in the image forming apparatus having the contact charging device, the following problems tend to arise.
Since the charge control agent of a dye type is soft and viscous, the charge control agent may be released from toner particles and transferred to the contact charging member, resulting in an increase in surface resistance, which tends to cause faulty charging Or faulty transfer.
Since the released charge control agent has a high chargeability, it may inhibit the toner particles from being electrostatically charged and at the same time make their fluidity poor, tending to cause blank areas in images and an uneven image density.